The present invention relates to a pulp screening device for separating good-quality fibers and foreign objects in paper pulp.
On the upstream side of a paper machine, there is provided a pulp screening device (pulp screen). The pulp screening device is a device for screening and separating good-quality fibers and foreign objects in paper pulp (i.e., a pulp suspension with a pulp density of 0.2 to 5%) with a screen cylinder thereof. Typically, the pulp screening device is equipped with one or two screen cylinders. First, the construction of a pulp screening device with a single screen cylinder will be described with reference to FIGS. 28 and 29. FIG. 28 shows a part-sectional plan view of a conventional pulp screening device. FIG. 29 shows a part-sectional side view taken in the direction of arrow D of FIG. 28.
A pulp suspension is fed to the pulp screening device by a pump. As illustrated in FIGS. 28 and 29, the pulp suspension flows in a tangential direction through the entrance 2 of a cylindrical container 17, and advances in an annular flow passage 4, formed by an inner casing 3 and the inner wall of the container 17. When the pulp suspension is circulating through the annular flow passage 4, heavy foreign objects such as sand, etc., are discharged outside the device from a trap 5 provided in the tangential direction opposite to the entrance 2, and the remaining pulp flows inside the inner casing 3 through the flow passage 4. Note that a cover 19 is provided on the upper surface of the container 17 so that the device can be operated under pressure.
A cylindrical screen cylinder 1 is disposed inside the inner casing 3. The upper portion of the screen cylinder 1 is fixedly attached to the inner casing 3, and this screen cylinder 1 partitions the inner side of the inner casing 3 into an agitation chamber 7 and an exit chamber 14. The pulp flowing in the flow passage 4 first flows in the annular agitation chamber 7 formed inside the screen cylinder 1.
A large number of slits of width 0.15 to 0.5 mm or holes of diameter 0.2 to 4.8 mm are provided in the peripheral surface of the screen cylinder 1, and the pulp is filtered and sorted by these slits or holes when flowing downward along the agitation chamber 7. That is, the good-quality fibers that can pass through the slits or holes in the peripheral surface of the screen cylinder 1 are discharged from an exit 9 via the exit chamber 14, while the foreign objects of sizes that cannot pass through the slits or holes in the screen cylinder, as they are, flow downward along the agitation chamber 7 and are discharged from a reject exit 10.
In addition, a rotor 6 is disposed within the agitation chamber 7. The rotor 6 is hung from the upper portion of a main shaft 11 and is equipped with a plurality of vanes 20 at equal spaces in the circumferential direction. The vane 20 is positioned, holing a predetermined space (2.5 to 8 mm) from the inner peripheral surface of the screen cylinder 1. The main shaft 11 is supported by bearings so that it is free to rotate, and is driven to rotate by an electric motor 13 through a V-pulley (not shown) mounted on the lower end portion thereof. If the rotor 13 rotates and therefore the vanes 20 revolve within the annular agitation chamber 7, the pulp suspension within the agitation chamber 7 is agitated. The foreign objects in the pulp are separated, and tangled fibers are untangled. As a result, clogging of the slits or holes in the screen cylinder 1 is prevented.
FIG. 30 shows how clogging of the slits or holes in the screen cylinder 1 is prevented by the vanes 20. As illustrated in FIG. 30A, the vane 20 revolves along the surface of the screen cylinder 1 at high speeds (10 to 30 m/s), holding a constant space from the cylinder surface. When the valve 20 is revolving, negative pressure is developed between the vane 20 and the screen cylinder 1, as shown in FIG. 30B. The suction force, developed by this negative pressure, causes the solution to flow backward into the agitation chamber 7 and therefore the tangled fibers or foreign objects, blocking holes 100 in the surface of the screen cylinder 1, are removed. After passage of the vane 20, the pulp suspension will flow from the agitation chamber 7 into the exit chamber 14 again, and the holes 100 in the screen cylinder 1 will be clogged with tangled fibers and foreign objects. However, the tangled fibers, etc., newly blocking the holes 100, are removed by the negative pressure produced by passage of the next vane 20. In the conventional pulp screening device, clogging of the holes in the screen cylinder 1 is prevented by repeating the aforementioned operation.
FIG. 31 shows a sectional view of the configuration of the hole 100 in the screen cylinder 1. The hole 100 is circular in shape, and a chamfered face 101 in the form of a dish is formed coaxially at the inlet of the hole 100 (on the side of the agitation chamber 7). When the vane 20 passes over the chamfered surface 101 in the surface of the screen cylinder 1, a turbulence (separating vortex) develops at the inlet of the hole 100, as shown by an arrow S in FIG. 31, and clogging of the hole 100 is suppressed by the turbulence S.
Furthermore, there are screen plates 1 of cross sections such as those shown in FIGS. 32 and 33. In the case of FIG. 32, trapezoidal grooves 111 are formed in the axial direction of the screen plate 1 (perpendicular to the paper surface) and forms a plurality of holes 110 at the bottoms of the grooves 33. In the case of FIG. 33, an axial waveform is formed on the peripheral surface of the screen cylinder 1, and a plurality of holes 120 are bored axially in the inclined portion 121 of the waveform. In any of the cross sections shown in FIGS. 32 and 33, revolution flow caused by the vane 20 develops a turbulence Sat the inlet of the hole, thereby preventing clogging of the hole.
Now, the construction of a pulp screening device with a double screen cylinder (inner and outer screen cylinders) will be described with reference to FIGS. 34 and 35. FIG. 34 shows a sectional view of the conventional pulp screening device with two inner and outer screen cylinders, and FIG. 35 shows a sectional view taken substantially along line Exe2x80x94E in FIG. 34. Note that the same reference numerals will be applied to the same parts as the aforementioned conventional pulp screening device having a single screen cylinder.
As illustrated in FIGS. 34 and 35, a pulp suspension flows in a tangential direction through the entrance 2 of a cylindrical container 17 and circulates through an annular flow passage 4. When the pulp suspension is circulating through the annular flow passage 4, heavy foreign objects such as sand, etc., are discharged outside the device from a trap 5 provided in the tangential direction of the flow passage 4, and the remaining pulp suspension flows from the flow passage 4 to inside an inner casing 3.
Cylindrical screen cylinders 1a and 1b are disposed inside the inner casing 3. These screen cylinders 1a and 1b partition the inside of the inner casing 3 into an agitation chamber 7 and exit chambers 14a, 14b. The pulp suspension flowing in the flow passage 4 first flows in the annular agitation chamber 7, formed between the screen cylinders 1a and 1b. When the pulp suspension is flowing downward along the agitation chamber 7, part of the pulp passes through the inner screen cylinder 1b and is filtered and sorted in the inner exit chamber 14b. The remaining pulp passes through the outer screen cylinder 1a, and is filtered and sorted in the outer exit chamber 14. On the other hand, the foreign objects of sizes that cannot pass through the screen cylinders 1a, 1b, as they are, flow downward along the agitation chamber 7 and are discharged from a reject exit 10.
In addition, within the agitation chamber 7, a plurality of outer vanes 20a are disposed in opposition to the outer screen cylinder 1a and a plurality of inner vanes 20b are disposed in opposition to the inner screen cylinder 1b. The vanes 20a, 20b are fixedly attached to a rotor 6 hung from the upper portion of a main shaft 11. The outer vanes 20a are disposed at equal spaces in the circumferential direction, holding a constant space (2.5 to 8 mm) from the outer screen cylinder 1a. Similarly, the inner vanes 20b are disposed at equal spaces in the circumferential direction, holing the constant space (2.5 to 8 mm) from the inner screen cylinder 1b. The main shaft 11 is freely rotatably supported by bearings and is driven to rotate by an electric motor (not shown) through a V-pulley 18 mounted on the lower end portion thereof. If the rotor 13 rotates and therefore the vanes 20a, 20b revolve within the annular agitation chamber 7, the pulp suspension within the agitation chamber 7 is agitated. The foreign objects in the pulp are separated, and tangled fibers are untangled. As a result, clogging of the slits or holes in the screen cylinders 1a, 1b is prevented.
The aforementioned pulp screening devices, however, have the following problems:
First, the conventional pulp screening device shown in FIGS. 28 and 29 has a limit to its processing ability since it has only a single screen cylinder 1. In addition, because of the configuration of the conventional vane 20, the revolution flow caused by the vane 20 becomes faster as it is near the surface of the vane 20 and slower as it is away from the vane surface. Therefore, the efficiency of cleaning the surface of the screen cylinder 1 is low, and there is a problem that the passage amount of the pulp will be reduced. Furthermore, the surface of the vane 20 remote from the surface of the screen cylinder 1 wastefully consumes the power required for friction, because it makes no contribution to the cleaning of the surface of the screen cylinder 1.
In the conventional pulp screening device shown in FIGS. 34 and 35, the speed of the revolution flow, developed by revolution of the vanes 20a and 20b, is slower at the inner screen cylinder 1b than at the outer screen cylinder 1a because of the difference in diameter between the inner and outer screen cylinders 1a and 1b. In addition, the pressure acting on the inner screen cylinder 1b is lower than that acting on the outer screen cylinder 1a because of a difference in centrifugal force. Therefore, the outer screen cylinder 1a tends to pass the pulp to more than the effective area of the screen cylinder 1a, whereas the inner screen cylinder 1b tends to pass the pulp to less than the effective area of the screen cylinder 1b. 
Because of this, when the quantity of pulp to be processed is excessively reduced, the outer screen cylinder 1a will pass the pulp therethrough, but there is a problem that the inner screen cylinder 1b will be liable to be clogged due to pulp flowing backward. Conversely, when the quantity of pulp to be processed is increased, the inner screen cylinder 1b will properly pass pulp therethrough, but there is a problem that the outer screen cylinder 1a will increase in passage resistance and will be likely to be clogged.
In addition, because revolution flow passes through between the inner and outer vanes 20b, 20a, the speed of the revolution flow within the agitation chamber 7 becomes faster only in the vicinities of the inner and outer vanes 20b, 20a and slower at positions away from the inner and outer vanes 20b, 20a. Because of this, the efficiency of cleaning the surfaces of the screen cylinders 1a, 1b is low and there is a problem that the quantity of pulp to be passed will be reduced. Furthermore, because of underagitation of pulp, a good quality of pulp will be discharged from the reject exit 10 without being processed by the screen cylinders 1a, 1b, and there is also a problem that the screening efficiency will be reduced.
In addition, as described above, the conventional pulp screening device has the problem that the quantity of pulp to be passed will be limited by clogging of the holes in the screen cylinder 1. The clogging of the holes in the screen cylinder 1 results from the configuration of the holes formed in the screen cylinder 1.
More specifically, the turbulence S (see FIGS. 31 to 33), developed at the inlet of the hole by the revolution flow resulting from revolution of the vane 20, has the effect of preventing the hole from being clogged. However, the strength of the turbulence S is affected by the configuration of the front edge of the hole (located on the upstream side of the revolution flow) In addition, the difficulty for tangled fibers to be caught, and the ease of removing foreign objects, are affected by the configuration of the rear edge of the hole (located on the downstream side of the revolution flow).
In the case of configuration such as that shown in FIG. 31, the turbulence S develops at the inclined surface, on the upstream side, of the hole 100 formed by the dish-shaped chambered surface 101, but the developed vertex S is weak because the inclined surface is gentle. Therefore, the turbulence S is less liable to reach the front edge 102 or rear edge 103 of the hole 100. Because of this, the effect of preventing clogging by the turbulence S is low. In addition, because the dish-shaped chambered surface 101 is formed coaxially with the hole 100, room for forming the dish-shaped chambered surface is required and the number of holes per unit area is thus limited. Because of this, there is a limit to increasing the quantity of pulp to be passed, by increasing the number of holes 100.
In addition, in the case of configuration such as the one shown in FIG. 32, the turbulence S which develops is strong, because the vertical portion of the trapezoidal groove 111 is located on the upstream side of flow. However, since the front edge 112 of the hole 110 is positioned at the groove bottom portion near the vertical portion of the trapezoidal groove 111, the vortex S developed is less likely to reach the front edge 112 and therefore the effect of preventing clogging of the hole 110 is low. Similarly, as the rear edge 113 is positioned at the groove bottom portion and is away from the inclined portion 114, separation of tangled fibers, etc, caught in the hole 100, is not easy. Besides, because the hole 110 can be disposed only in the bottom portion of the trapezoidal groove 111, the number of holes per unit area is also limited.
Furthermore, in the case of configuration such as that shown in FIG. 33, the turbulence S develops at the vertex of the waveform formed on the surface of the screen cylinder 1. However, the front edge 122 of the hole 120 is far from the vertex of the waveform and the front and rear edges 122, 123 are at the inclined portion 121 of the waveform. Therefore, the turbulence S is less likely to reach the edges 122, 123, and the effect of preventing clogging of holes by the turbulence S is thus low. In addition, since the rear edge 123 has an acute angle, separation of a lump of pulp, etc., caught on the edge, is not easy. Moreover, the number of holes per unit area is limited, because the hole 120 can be disposed only in the inclined portion 121 of the waveform.
As described above, in any of the hole configurations shown in FIGS. 31 to 33, the effect of preventing clogging by the turbulence S is not satisfactory. Therefore, it is necessary to make the turbulence S stronger by revolving the vanes 20 at high speeds in order to prevent clogging of holes. The power required for revolving the vanes 20, however, becomes greater in proportion to the square to cube of the revolution speed, so the quantity of passage per consumption power is inversely reduced.
The present invention has been made in view of the problems found in the prior art. Accordingly, it is the primary object of the present invention to provide a pulp screening device that is capable of screening a large quantity of pulp with low power, by preventing clogging of a screen cylinder.
To achieve this end and in accordance with one important aspect of the present invention, there is provided a pulp screening device, comprising:
a pair of inner and outer screen cylinders; and
one or a plurality of vanes which revolve within an agitation chamber formed between the inner and outer screen cylinders, holding a predetermined small space from each of the inner and outer screen cylinders.
The agitation chamber can be practically partitioned in the circumferential direction, by providing the vanes which revolve within the agitation chamber formed between the inner and outer screen cylinders, holding a predetermined small space from each of the inner and outer screen cylinders. With this arrangement, the internal pressure within the agitation chamber becomes higher, as the revolution speed of pulp is increased. Therefore, the separation and agitation of foreign objects and lumps of pulp are accelerated, and clogging of the screen cylinders is prevented and the quantity of pulp to be passed is increased. In addition, the distance between the inner and outer screen cylinders can be shortened by sharing a single vane with the inner and outer screen cylinders. Because of this, the speed difference of the pulp between the inner and outer screen cylinders caused by the difference in diameter therebetween, and the pressure difference caused by centrifugal force, become smaller compared with prior art. Particularly, a reduction in the quantity of pulp to be passed due to clogging of the inner screen cylinder is prevented. Therefore, there is no possibility that the screen cylinders will be clogged even when the revolution speed of the vanes is relatively slow, and there is obtained an effect that a large quantity of pulp can be screened with low power.
In a first preferred form of the present invention, the revolution-direction front portion of the vane has a wall face extending radially toward the peripheral surfaces of the inner and outer screen cylinders. With this arrangement, the direction of the revolution flow of the pulp is changed from the circumferential direction to the radial direction by the wall face. The radial flow of the pulp renders it possible to partition the agitation chamber efficiently.
In a second preferred form of the present invention, the wall face is formed at a right or acute angle to the direction of revolution. With this arrangement, the revolution flow of the pulp can perpendicularly approach the peripheral surfaces of the inner and outer screen cylinders, and it becomes possible to partition the agitation chamber more efficiently.
In a third preferred form of the present invention, the cross section of the vane is formed so that the spacing between the cross section and each of the inner and outer screen cylinders widens gradually from the wall face in the direction of revolution. With this configuration, the pressure within the agitation chamber becomes negative on the rear portion side of the vane. Therefore, the pulp suspension flows backward from outside the inner and outer screen cylinders into the agitation chamber. As a result, lumps of pulp, etc., caught in the screen cylinders, are removed. In addition, the pulp density within the agitation chamber is diluted, and there is obtained an effect that repassage of the high-density pulp, which is not passed through the screen cylinders, becomes easy.
In a fourth preferred form of the present invention, the cross section of the vane is formed into the shape of a wedge extending at an acute angle from a revolution-direction tip end to both proximity portions closest to the inner and outer screen cylinders. With this shape, the position of the tip end of the vane can be adjusted by adjusting the incidence angle of the vane, and it becomes possible to supply pulp to the inner and outer screen cylinder equally.
In a fifth preferred form of the present invention, a distance from the tip end to both proximity portions is set to two to five times a distance between both proximity portions With this, there is no reduction in the screening efficiency of the screen cylinder and no rise in the operating power per unit processing ability of the screen cylinder. Therefore, clogging of the inner and outer screen cylinders is prevented, whereby it becomes possible to assure a large quantity of pulp to be passed with low power.
In a sixth preferred form of the present invention, the aforementioned tip end is disposed at a center between the inner and outer screen cylinders, or at a position offset from the center toward the outer screen cylinder. With this arrangement, the load for processing pulp can be balanced between the inner and outer screen cylinders.
In a seventh preferred form of the present invention, the cross section of the vane is formed so that the spacing between the cross section and each of the inner and outer screen cylinders widens gradually from both proximity portions in the direction of revolution. With this configuration, the pressure within the agitation chamber becomes negative on the rear portion side of the vane. Therefore, the pulp suspension flows backward from outside the inner and outer screen cylinders into the agitation chamber. As a result, lumps of pulp, etc., caught in the screen cylinders, are removed. In addition, the pulp density within the agitation chamber is diluted, and there is obtained an effect that repassage of the high-density pulp, which is not passed through the screen cylinders, becomes easy.
In an eighth preferred form of the present invention, adjacent vanes of the aforementioned plurality of vanes are connected by a partition wall. This further divides the agitation chamber into two parts. Therefore, flow from inside the agitation chamber to outside the agitation chamber, which is caused by centrifugal force, can be blocked, and it becomes possible to increase the quantity of pulp to be passed at the inner screen cylinder.
In a ninth preferred form of the present invention, the cross section of an inner discharge tube at a point where the inner discharge tube joins an outer discharge tube is set greater than the cross section of the outer discharge tube, pulp being passed through the inner screen cylinder and flowing in the inner discharge tube and also being passed through the outer screen cylinder and flowing in the outer discharge tube. With this setting, an effect is obtainable that the flow of the pulp from the inner discharge tube becomes satisfactory and that the quantity of pulp to be processed is thus increased.
To achieve the aforementioned object and in accordance with another important aspect of the present invention, there is provided a pulp screening device, comprising:
a screen cylinder; and
one or a plurality of vanes which revolve within an agitation chamber formed outside or inside the screen cylinder, holding a predetermined small space from the screen cylinder;
wherein a revolution-direction front portion of the vane has a wall face extending radially toward the peripheral surface of the screen cylinder, and the vane is formed so that the spacing between the vane and the screen cylinder widens gradually from the wall face toward a revolution-direction rear end.
With such a construction, clogging of the screen cylinder can be prevented by making the difference in pressure within the agitation chamber greater before and after the wall face, and there is obtained an effect that a great quantity of pulp can be screened with low power.
To achieve the aforementioned object and in accordance with still another important aspect of the present invention, there is provided a pulp screening device, comprising:
a screen cylinder having a plurality of filter holes; and
one or a plurality of vanes which revolve within an agitation chamber formed outside or inside the screen cylinder, holding a predetermined small space from the screen cylinder;
wherein a plurality of conical hollows are provided in the peripheral surface of the screen cylinder which faces the agitation chamber, and the filter hole is formed to be offset from the center of the conical hollow in the direction opposite to the direction in which the vane revolves.
With construction like this, a strong, turbulence is developed at the inlet of the filter hole by the revolution flow of the pulp, and the pulp is satisfactorily agitated. In addition, a lump of pulp and foreign objects are prevented from being caught in the filter holes, and clogging of the filter holes is prevented. Therefore, there is obtainable an effect that a large quantity of pulp can be screened with low power.